For the purpose of improving the performance of a photovoltaic device such as a solar battery, it is important to ensure the following three features, which are to efficiently introduce sunlight into a device, to convert introduced optical energy into electric energy by high current-voltage characteristics, and to efficiently draw converted optical energy to outside. It is also important to ensure long-term reliability that the photovoltaic device can keep outputting power for a long time in a case of additionally considering the practical use of the device.
Generally, to produce the photovoltaic device, an impurity diffusion layer (hereinafter, “diffusion layer”) is formed on the surface of a substrate by diffusing impurity opposite in conduction type to the substrate and a PN junction is formed.
The photovoltaic power of the photovoltaic device largely depends on the difference in level between conduction types. Accordingly, from the viewpoint of the photovoltaic power, an impurity concentration (a dopant concentration) that determines a conduction level is preferably high. In addition thereto, the diffusion layer functions as a part of electrodes for efficiently drawing a generated current to an external circuit. From this viewpoint, the dopant concentration is preferably high.
Furthermore, a resistance component gradually increases in a portion in which the electrodes are connected to the PN junction as the photovoltaic device is used for a long time. Accordingly, in the case of additionally considering the long-term reliability, the dopant concentration is preferably high with a view of suppressing the influence of such an increase in the resistance component.
Meanwhile, silicon semiconductor indicates a higher crystal quality as the impurity concentration present inside the silicon semiconductor is lower. When the dopant concentration is too high, the crystal quality as the semiconductor greatly degrades and a recombination rate accelerates, resulting in the reduction in the photovoltaic power. Therefore, when the PN junction is a single junction structure, it is important to set the dopant concentration to an appropriate value while keeping the balance from the three viewpoints mentioned above.
Furthermore, there is a “selective emitter” as one of effective and practical methods for the above objects although its PN junction structure is not a single junction structure. This is a method for changing concentration settings of the diffusion layer depending on structural features. As the most typical example, there is known a two-stage structure, that is, a region near a portion just under each electrode is set as a heavily doped region where the dopant concentration is high and the other region is set as a lightly doped region where the dopant concentration is low (see, for example, Patent Literatures 1 and 2).
In the above example, the heavily doped region near the portion just under each electrode is responsible for a function as a part of the electrode, and the other lightly doped region is responsible for a function of suppressing the degradation in the crystal quality as the semiconductor. That is, it is intended to improve the current-voltage characteristics of a solar battery cell by assigning expected roles mainly to the respective regions.